The present invention relates to a thin film transistor with the use of a semiconductor film made of an aggregate of organic semiconductor molecules, and a manufacturing method therefor, particularly the thin film transistor characterized by having a controlled pattern shape of an electrode by improving an orientation order of the organic semiconductor molecules making up a semiconductor film with the use of a selectively disposed self assembled monolayer film, a manufacturing method therefor, and a visual display unit using it.
Recently, an active matrix type display unit using an active device represented by a thin film transistor (TFT) expands its market, while being used for not only a notebook-sized personal computer or a mobile telephone but also a monitor of a workstation or television, by making use of advantages of the excellent picture equivalent to that of a Cathode-Ray Tube (CRT), a low power consumption and a saved space compared to the CRT. However, the active matrix type display unit is more expensive than the CRT, and it is necessary to further lower the price for wider spreading. As one of methods for lowering the price, an application of the organic thin-film transistor (TFT) which can be manufactured by a simple and easy method to the active device, is under examination. A plasma chemical vapor-deposition (CVD) apparatus used for manufacturing an insulation film or a semiconductor film of a current amorphous silicon TFT, and a sputtering apparatus used for manufacturing an electrode, are expensive. The CVD process has a low throughput, because of requiring a high temperature of 250-300xc2x0 C. for forming the film and a long time for maintenance such as cleaning. A vacuum evaporation apparatus used for manufacturing the organic TFT or a coating device is more inexpensive than the CVD apparatus and the sputtering apparatus, has the low film-forming temperature, and is easy for maintenance. Accordingly, the application of the organic TFT to the active matrix liquid-crystal display may promise a significant cost reduction.
A typical organic TFT is made of a substrate, a gate electrode, an insulation film, a drain electrode, a source electrode, and an organic semiconductor film; and performs a switching operation by modulating an amount of carriers accumulated in an interface between the insulation film and the organic semiconductor film from an accumulation state to a depletion state, with applied voltage to the gate electrode, to change an amount of the current passing between the drain electrode and the source electrode. The organic semiconductor film consists of an aggregate of the organic semiconductor molecules consisting of low-molecules or macromolecules. Known monomeric materials include an acene-based material represented by pentacene or thiophene oligomer, while polymeric materials include poly-3 and hexyl thiophenes (P3HT) which belong to polythiophenes and have a regioregular (having such an array as the whole chain forms a line in a same direction and the head and the tail are connected) structure of a highly regular nanostructure; a copolymer of fluorene-bi-thiophene (F8T2) as a polyfluorene system; and polyphenylene vinylene (PPV).
In order to obtain a high switching operation by using the organic semiconductor films, the organic semiconductor film formed on the surface of the insulation film needs to have all the organic semiconductor molecules oriented in the same direction and disposed densely.
As a method for forming the organic semiconductor film having the highly oriented order, a method is known in which the surface of the insulation film is previously coated with a self assembled monolayer film (SAM). For instance, as described in Applied Physics Letters 81(23), pp. 4383-4385 (2002) by A. Salleo and others, when the surface of the insulation film comprising a thermally oxidized film of silicon is coated with a SAM such as octadecyl trichlorosilane (OTS), the orientation order of the organic semiconductor film consisting of the F8T2 semiconductor macromolecules is improved, which leads to improvement of the switching performance. In addition, as described in IEEE Trans. Electron. Devices, 44, pp. 1325-1331 (1997), by Y. Y. Lin and others, it is reported that when the surface of the insulation film comprising the thermally oxidized film of silicon is coated with OTS, the orientation order of the organic semiconductor film consisting of the semiconductor low molecules of pentacene deposited by vacuum evaporation is improved, and crystal grain sizes of the organic semiconductor film deposited on the OTS coated thermal oxide film is greater than those deposited on the thermal oxide film without OTS coating, which leads to improvement of the switching performance of the TFT. In addition, as described in SCIENCE Vol. 280, pp. 1741-1743 (1998) by H. Sirringhaus and others, it is known that the electric field-effect mobility of the formed organic semiconductor film is improved to 0.01-0.1 cm2/Vs, when the surface of the insulation film is previously coated with a SAM consisting of hexamethyldisilazane, and then with P3HT thereon.
As described above, it is reported that a method for forming an organic semiconductor film on the surface of an insulation film previously modified with SAM, improves an orientation order of the organic semiconductor film and a switching performance, and provides an organic TFT having a performance equal to or better than that of a current TFT which uses an inorganic semiconductor of amorphous silicon for the semiconductor film.
When the thin film transistor is used in a display device using a backlight, such as a liquid crystal display, a light leakage current caused by photoirradiation from the back side of a substrate should be small to show an adequate switching performance as the thin film transistor. The light leakage current is generated because the semiconductor film absorbs the light applied from the back side of the substrate in a state that a voltage causing the depletion of carriers in the interface between the semiconductor film and the insulation film is applied to the gate electrode, and forms photocarriers which travel due to the voltage applied to drain/source electrodes. It is known that the amount of the leak current is large, when a travelling property of the carriers is high in the semiconductor film formed in the region outside the gate electrode-projected region of the surface of the insulation film, which is not shaded by the gate electrode, and directly absorbs the light applied from the back side of the substrate, particularly in a potential floating region which is not included even in the drain/source electrode region. It is described in detail, for instance, in Display and Imaging Vol. 7, pp. 129-135 (1998) by Wakagi and others.
As described above, conventional coating with a SAM on the surface of the insulation film to be a substrate has a problem of the increase in the light leakage current, while improving the order of orientations of organic semiconductor molecules in an organic semiconductor film, and a switching performance such as electric field-effect mobility and an on/off ratio of the current. This is because the coating of a SAM has improved the orientation order of the organic semiconductor molecules in the organic semiconductor film not only formed in a gate electrode-projected region on the surface of the insulation film, which controls a switching performance such as an electric field-effect mobility and an on/off ratio of the current, but also formed outside the gate electrode-projected region which controls a light leakage current.
Modification of the surface of the insulation film with a SAM has another problem of an adverse effect on a coating process for the electrode. There has been a problem that a method of forming an electrode by applying an electroconductive ink to the surface of the insulation film and burning it, can not finely form a thin film transistor, because the wettability of the electroconductive ink against the surface of the insulation film affects pattern accuracy of the electrode in the method, a SAM generally having water repellency rejects the electroconductive ink, and makes the pattern accuracy of the electrode low.
As described above, in an organic thin film transistor, the modification of the surface of the insulation film with a self assembled monolayer film, for improving the orientation order of the organic semiconductor molecules and improving the switching performance, has a problem of increasing the light leakage current and being incapable of finely forming the electrode with a coating process.
To solve the above problem, according to the invention, a thin film transistor is provided having a gate electrode, a gate insulation film, a source electrode, a drain electrode, a semiconductor film and a protection film, stacked on a substrate, in which the semiconductor film is composed of an aggregate of organic semiconductor molecules, and the orientation order of the organic semiconductor molecules of the semiconductor film formed in a gate electrode-projected region on the surface of the insulation film is higher than that of the semiconductor film formed outside the region. In particular, a method for forming the organic semiconductor film having the above characteristics is provided which comprises selectively disposing a self assembled monolayer film on the surface of the insulation film and then forming an organic semiconductor film thereon by making use of the property that the orientation order of the organic semiconductor film formed on the self assembled monolayer film is improved. As the above thin film transistor is used a thin film transistor which has a self assembled monolayer film in an interface between a semiconductor film formed on the surface of an insulation film in a gate electrode-projected region and the insulation film, but not in an interface between the semiconductor film formed outside the region and the insulation film. As a self assembled monolayer film is used a water repellent monolayer film which has a carbon chain partly terminated with a fluorine or hydrogen atom.
In addition, a thin film transistor may be used having drain/source electrodes formed by applying and burning an electroconductive ink from a metal, a metallic oxide or an electroconductive polymer in a form of ultra-fine particles, a complex or a polymer capable of forming a liquid material by dispersing into a solvent. Further, a thin film transistor may be used which is formed by stacking a gate electrode, a gate insulation film and a self assembled monolayer film sequentially on a substrate, the self assembled monolayer film being selectively disposed in a gate electrode-projected region, and has a source/drain electrode formed near the both ends of the pattern of the self assembled monolayer film by applying and burning an electroconductive ink including at least one of to ultra-fine metal particles, a metal complex or a electroconductive polymer.
A method is provided for manufacturing a thin film transistor which comprises irradiating a self assembled monolayer film formed on the surface of an insulation film with a light from the surface side of a substrate through a photomask, to remove the self assembled monolayer film from a region outside a gate electrode-projected region in which a semiconductor film is to be formed. In addition, a method may be employed which comprises irradiating a self assembled monolayer film formed on the surface of an insulation film with a light from the back side of a substrate using a gate electrode as a photomask to remove the self assembled monolayer film from a region outside a gate electrode-projected region in which a semiconductor film is formed. Further, a method may be used which comprises pressing a self assembled monolayer film applied on a smooth substrate to the surface of the insulation film to selectively transfer the self assembled monolayer film only onto the gate electrode-projected region by making use of a step between the gate electrode-projected region of the surface of the insulation film and other regions.
An active matrix type thin film transistor substrate is also provided which has several gate electric bus lines, an insulation film, several signal bus lines intersecting with the gate electric bus lines in a matrix form, a protection film, and a pixel electrode, in which the thin film transistors of the present invention are disposed at the intersections of the several gate electric bus lines with the signal bus lines, the gate electric bus line is connected to the gate electrode, the signal bus line is connected to a drain electrode, and the pixel electrode is connected to a source electrode. Further, an active matrix drive display unit is provided using an active matrix type thin film transistor substrate for driving a liquid crystal device or an electrophoretic device.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings.